Lobbs Farm Shop has the great benefit of being on-site at The Lost Gardens of Heligan (you don't have to pay the entrance fee to reach it though), so you not only get to buy fresh local produce, but do so in a delightful setting. The shop is a bright, purpose built "shed" in the car park area, easy to access and pleasant to shop in.
As well as fruit & veg, the shop sells meat, dairy and deli' products. Many of the last group are pretty high-end and can be pricey - after all, they're appealing to the tourist trade as much as providing a local service - but it certainly makes for a good range.
As a veggie I'm unable to comment on that are of their service, but the meat counter has always had a small queue of people, whom a small amount of eaves-dropping suggest are regulars who seem to be getting a friendly and helpful service. This would certainly be in-keeping with the quality of service I received from the cheese and veg sections.
There's a good range of local and more distantly sourced cheeses - clearly labelled so you know which is which. I can recommend the yarg with wild-garlic wrapping.
In the deli' section you can buy local beers, speciality snacks, sauces, ready-made pies and all sorts of other goodies.
The fruit and veg' section does sell a lot of produce that's not particularly local, but does provide this information for every item, so that you can choose to limit your food-miles.
There's also a small bakery area... but as a resident of nearby Mevagissey, I can tell you that the baker in question is easily found in the centre of the village, and is worth a visit if you're in the area!
There is a walk between Heligan and Mevagissey, but it's about 3KM and one's at the top of the hill, and the other at the bottom, so you might prefer to use the ample parking... but if you're staying in the village it's a gorgeous walk and helps with those food miles.